1. Field of the Invention
The present invention relates to a noise removal circuit and a noise removal method.
2. Description of the Related Art
Prior art related to signal noise removal are covered in signal recording and reproduction devices such as a hard disk and an optical disk. For instance, optical disks that allow data recording using laser beam are classified into write-once optical disks such as CD-R, DVD-R and DVD+R and rewritable optical disks such as CD-RW, MD, DVD-RAM, DVD-RW, DVD+RW and MO.
A recording and reproduction device for such optical disks (hereinafter written as an “optical disk device”) comprises, as an example, an optical pickup, a WBL (wobble signal) detection unit, an LPP detection unit, a write clock generation unit, a decoder, a spindle motor, a spindle servo circuit, an optical pickup servo circuit, a processor, an interface unit, an encoder, a laser control unit and a ROM.
Of all the components, the aforementioned WBL signal extraction circuit is shown in FIG. 11. An optical pickup 11 irradiates an optical beam onto the recording track of an optical disk 1 based on a laser control signal. A WBL detection unit 12 comprises a wobble (WBL) signal B.P.F. (Band Pass Filter) circuit 12a, a comparator 12b and so on. A radial push-pull signal SDT, detected based on a reflected light of an optical beam B, is input to the wobble signal B.P.F. circuit 12a. The wobble signal B.P.F. circuit 12a removes a high-frequency noise component and extracts a WBL signal component A_WBL from the radial push-pull signal SDT (See, e.g., Japanese Patent Application Laid-open Publication No. 2000-293855).
A binary-valued WBL signal is output by comparing the WBL signal component A_WBL (negative side) and a reference voltage (positive side) using the comparator 12b. The binary-valued WBL signal is output to the write clock generation unit, the spindle servo circuit 17 and so on.
In extracting the WBL signal component A_WBL, the wobble signal B.P.F. circuit 12a removes the high-frequency noise component from the radial push-pull signal SDT. Nevertheless, some noise component has remained in the WBL signal component A_WBL after passage through the wobble signal B.P.F. circuit 12a. 
Jitter occurs in the clock during optical disk recording or reproduction in the event of an increase in this noise component. Occurrence of the jitter has hindered synchronization of the optical disk rotation speed and the clock, resulting in recording or reproduction failure and other problems.
The wobble signal may become amplitude-modulated depending on the disk or pickup condition, occasionally resulting in the wobble signal being impossible to read correctly.
Further, the wobble signal may be lost as a result of disk damage or other cause, occasionally adversely affecting the control system using the wobble signal.
Still further, an AC signal may be superposed onto the wobble signal at disk rotation cycles due to effects such as unevenness or double refraction in disk protective film and pickup condition, occasionally resulting in the wobble signal being difficult to read correctly.